Just a few more notes:

Zda tyto věta jsou korektní?

There are two things wrong in this sentence. The word korektní is not the same thing as correct/right. Korektní in Czech is either used in the sense of political correctness OR in the sense of polite and right, for example in a business or when describing a behaviour during a situation where some standards are expected from you. The other problem is a  wrong way to construct a question. Using "zda" sounds quite formal (people use "jestli" more often) and it is used in compound sentences, like "Chtěla bych se zeptat, zda jsou tyto věty správně." The rest is the word order. "Jsou tyto věty správně?" is the simplest and correct way to ask the question.

Ráda piji čaj/kávu. "Piji" is not exactly incorrect, it is just rarely used and viewed as formal or archaic by many. Kávu is a matter of declination.

Ve dvanáct hodin už spím. "Ve" is correct, as dvanáct begins with a consonant. Yeah, Czech is a language full of consonants and tricky pronunciation of their groups (words like krk, prst,...) but even we have some limits ;-). The same -e addition applies to other prepositions too, z/ze, s/se, k/ke

Jsou tyto věty správně?